Most important output of pore pressure-deformation analyses

[slam00000]

Hello All

I have done staged construction deformation analyses of earth/tailings dams using advanced numerical modelling ( no ready FOS can be given) . I can get all kinds of output(related to hydrology ore pressures,saturation, flow velocity.. and mechanical outputs: stresses , strains, displacemnets,void ratio....
My objective is to investigate the stability of the dams during staged construction
These are the outputs that I beleived ultimately will dictate the safety of the system

1- I took some horizontal profiles and plot the settelment along them to see if there are remarkable differential movements that could cause vertical cracks

2- I took vertical profiles at each satge and plotted
a: the horizontal displacement to see if there is big differential movement that could lead to horizontal cracks
b: the operating friction angle (t/P'):where t is the operating shear stress , and p' is effective pressure) and compare it with the stregnth friction angle to check the FOS at each point along the considered profiles (my materials are almost cohesionless)

DO you recommend me to plot any other outputs that are practically important to evaluate the response of the dam during construction.. and why..?

 

[slam00000]

Thanks for that
I just report the Horizontal permeability which is in cases (beach and slimes), which I assumed it to be 10 times higher than the corresponding vertical permeability.
Also my permeabilities are im m/sec.
The results of vertical permeability I got from literature.
a tailings dam is really a difficult system

 

[slam00000]

Hello dgillette and all
Quick question I forget to ask about: I do not know why most of the stability and consolidation analyses talk about excess pore pressure rather than the total pore pressure. high structures such as tailings dams containing slurries with huge water content, the pressure of the weight of the fluid should not be overlooked. How can we trust stress-deformation analyses that implement only the excess pore pressure obtained from the consolidation analysis. This is not a one dimentional consolidation test in which one can ignore the gravity-induced pore pressure.
Thanks for your time

 

[dgillette]

If the slimes are too thin to be handled by conventional soil mechanics, why not apply the fluid pressure as a surface pressure on the model?

We tend to think in terms of excess PWP in large part because that is what governs consolidation, and for most of us, the 'static'/steady-seepage component of PWP is implicit in stability/deformation analysis - implicit in the sense that we automatically put in a phreatic line (regardless of whether hydrostatic is the correct way to model the static PWP).

____________________________________

FYI - See Chapters 6 and 7 for typical tailings dam practice, although it was written specifically for coal:

http://www.msha.gov/DesignManual/DesignManual.asp

 

[Dirtguy4587]

Typically in the Oilsand mines (and I have worked on dykes with a very similar zonation to yours), the excess pore pressures of the foundation soils governs the stability (in a limit-equilibrium sense), and impacts the FoS greatly.

Also, one comment on your work. Have you considered static liquefaction contributing to an upstream failure of the shell of the dyke. If the upstream beach (the slimes) are deposited in a loose state (i.e. deposited as beach below water) there is high potential that this zone is contractive, and may liquefy when the subsequent "shell" lifts are placed. May be something to add to your study if required.

 

[dgillette]

He's trying to do a fully coupled analysis that allows for excess PWP due to shearing and consolidation, which should (I think) take care of what most of us would call static liquefaction.

See approx. Jan 14. I believe reaching the Sladen instability line referred to is essentially the same as static liquefaction.

 

[slam00000]

Dear Dirtguy4587 and dgillette
My foundation soil is highly overconsolidated clay (very stiff) with very low permeability.
The shell (embankment dykes) is compacted dense materials. Its upstream portion rest on the loose beached tailings materials. I have analyzed two systems
1-Gold ( its beached tailings has relatively moderate coefficient of consolidation and permeability or around
10E-06
2-Oil Sand (its beached tailings has relatively low coefficient of consolidation and permeability or around 10E-08).
Both facilities have the same drainage and starter dyke systems. Also, the shell in both facilities has almost the same permeability (10E-03) and strength Phi=33 degree.
The results show that in oil sand the beached tailings zone under the upstream shell demonstrate undrained behavior . I’ve made this judgment based on this observation: during the staged construction, the rate of pore pressure increase with time is observed to be much higher than the rate of effective stress increase with time at different points in this zone).
The gold system however shows drainage behavior.
This difference in the drainage condition dramatically influences the stability. For example, I have found that when the height of the oil sand exceeds 30 m, the increase rate of the maximum horizontal displacement in oil sand is 0.6 m/year and in gold is 0.05 m/year and when the height reaches 40 m the max horizontal displacement in the oil sand reaches to 2 m and 0.4 m in the gold.

Thanks